The invention relates to a televisioncamera comprising at least one pick-up tube and a control circuit for the intensity control of an electron beam present in the pick-up tube, the electron beam being generated by an electron gun comprising at least a cathode and a control electrode, an output, carrying a picture signal, of the pick-up tube being connected for beam current intensity control to a control input of the control circuit, an output of which, carrying a control signal, being connected to the control electrode in the pick-up tube, the control circuit comprising a setting stage for an adjustment of the beam current intensity and a control stage for an automatic control thereof.
Such a camera is described as a prior art camera in German Auslegeschrift No. 24 24 402. It is described that the picture signal generated by the pick-up tube is subjected to the following processes in the control circuit: the picture signal is first amplified, thereafter a black level in the picture signal is clamped on a reference level by means of a clamping circuit and then that signal portion which exceeds a nominal peak-white value is separated from the picture signal and, via a low-pass filter in which the smoothed signal is superimposed on a d.c. voltage for adjustment, applied to the control electrode of the electron gun in the pick-up tube.
It is described that a drawback of the prior art camera is that oscillations can and will in practice frequently occur due to the feedback. As regards the cause or possible several causes, respectively, of the oscillations, it is remarked that an exact indication cannot be given. Finally the described camera is said to be highly instable and unsuitable for practice.